Controlling air circulation in a data center

ABSTRACT

A method of controlling air circulation in a data center system. The data center system includes: a cold aisle; a hot aisle including a floor element and a ceiling element; a server rack comprising a controller and servers separating the cold aisle from the hot aisle; and an air conditioning unit. The method includes: toggling, by the controller, the hot aisle between: (a) a first configuration in which the ceiling element is opened and the floor element is closed and (b) a second configuration in which the ceiling element is closed and the floor element is opened, wherein the first configuration enables circulation of air from the from the hot aisle to the cold aisle via the air conditioning unit, and wherein the second configuration enables circulation of air from the from the hot aisle to the cold aisle without the circulated air passing through the air conditioning unit.

FIELD OF THE INVENTION

The present invention relates to a data center system in which hot andcold aisles are separated by server racks.

The present invention further relates to a server rack for such a datacenter system.

The present invention yet further relates to a method of controlling thetemperature in a data center.

BACKGROUND

Data centers are large computer rooms that are organized in so-calledhot aisles and cold aisles. Server racks separate the hot aisles fromthe cold aisles, with the servers in the server racks typically arrangedto draw cold air from the cold aisles to cool the servers in the racksand expel the heated air into the hot aisles. A computer room airconditioning (CRAC) unit draws in air from the hot aisles, typicallythrough ceiling tiles of the computer room and returns the cooled air tothe cold aisles via perforated floor tiles and/or grates in the coldaisles. The conduits required for the air flow from and to the CRAC unitare typically defined by a suspended ceiling and a raised floor, as iswell-known per se.

Data centers require large amounts of energy to operate. A substantialpart of this energy is being consumed by the CRAC unit. As iswell-known, the efficiency of air conditioning units such as CRAC unitsdepends on the temperature of the inbound air, i.e. the air drawn fromthe hot aisles. In case of low server activity, the air of theassociated hot aisle may have a relatively low temperature, whichtherefore negatively impacts on the efficiency of the CRAC unit. This isundesirable, as this pushes up the energy consumption of the data centeras a whole, which not only increases the operation cost of the datacenter but also increases the risk of black-outs if the energy demand ofthe data center cannot be met, which is highly undesirable. It istherefore desirable to try and improve the energy efficiency of datacenters.

U.S. Pat. No. 8,156,753 B2 discloses a data center cooling solutionproviding techniques for using baffles, doors and roof sections toprevent warm air from being entrained into a cold aisle in a datacenter, wherein the data center generally contains an air cooling systemand a raised floor structure. The raised floor structure is configuredto deliver cool air into the data center through a plurality of gratesand perforated tiles in the floor. Electronic equipment racks aredisposed around the grates and perforated tiles, such that the frontfaces of the equipment racks face the grates and perforated tiles. Acollection of baffles, doors or roof sections inhibit the mixing of thecool air delivered by the air cooling system and the warm air exhaustedby the electronic equipment. This improves cooling of the servers andtherefore increases the temperature gradient between the hot aisles andcold aisles. However, this solution does not address the problem of CRACinefficiency if this temperature gradient is small, e.g. because of lowserver activity.

US 2011/0303406 A1 discloses a computer room air-conditioning systemincluding a temperature detection unit which is provided for each of afront and a back of each rack, and which measures air temperatures atthe front and the back of the rack. A control device for acquiring ameasured temperature by each temperature detection unit and forperforming control based on the measured temperature is also present.With the configuration, the control device includes a temperaturedifference calculation unit for calculating a temperature differencebetween cool air at the front and warm air at the back of each rackbased on each measured and acquired temperature; and a heating element,i.e. server, cooling control unit for controlling by adjustment anamount of flow of cool air from the underfloor space to the computerroom based on the calculated temperature difference. This improves theenergy efficiency of the data center because the air flow to and fromthe CRAC unit is reduced for small temperature gradients. Nevertheless,this still requires the CRAC unit to operate at reduced efficiency,albeit at reduced air volumes.

BRIEF SUMMARY OF THE INVENTION

The present invention seeks to provide a data center system that can beoperated at further improved energy efficiency.

The present invention further seeks to provide a server rack for use insuch a data center system.

The present invention yet further seeks to provide a method ofcontrolling the temperature in a data center.

According to an aspect of the present invention, there is provided datacenter system comprising a cold aisle comprising a first perforatedfloor element; a hot aisle comprising a second perforated floor elementand a ceiling element that each can be opened and closed; a server rackcomprising a plurality of servers separating the cold aisle from the hotaisle; an air conditioning unit having an input coupled to the hot aislevia the ceiling element and an output coupled to the cold aisle via thefirst perforated floor element; and a controller arranged to toggle thedata center system between a first configuration in which the ceilingelement is opened and the second perforated floor element is closed; anda second configuration in which the ceiling element is closed and thesecond perforated floor element is opened.

In this system, the air conditioning (AC) unit can be bypassed if thereis no need to feed the air from the hot aisle to the AC unit. In thisbypass mode, the air from the hot aisle is recirculated to the coldaisle via the raised floor including the first perforated floor elementand the adjustable second perforated floor element. The floor elementsmay for instance be tiles. This has the advantage that the AC unit canbe kept in a standby mode in situations where the AC unit wouldotherwise have operated at reduced efficiency, thus improving the energyefficiency of the data center system.

The controller may be arranged to toggle between the first configurationand the second configuration upon the temperature in the cold aislereaching a defined threshold, such as 28° C. If the temperature in thecold aisle exceeds this defined threshold, cooling of the servers in theserver racks may be compromised, such that upon reaching thistemperature the AC unit should be included in the recirculation loop toensure that the air temperature in the cold aisle does not exceed thistemperature.

Alternatively or additionally, the controller may be arranged to togglebetween the first configuration and the second configuration upon thetemperature in the hot aisle reaching a defined further threshold, suchas 45° C. This further threshold may be selected based on the efficiencycharacteristics of the AC unit, such that the AC unit is only activatedwhen it can operate in a sufficiently efficient manner.

The server rack may comprise a temperature sensor coupled to thecontroller, said controller being responsive to said temperature sensor,which may be located at the front of the server rack to sense thetemperature in the cold aisle or at the back of the server rack to sensethe temperature in the hot aisle. The server rack may comprise a pair ofsuch temperature sensors, i.e. a temperature sensor at the front of therack and a further temperature sensor at the back of the rack, with thecontroller being responsive to each of said temperature sensors.

In a particularly advantageous embodiment, the controller is integratedin the server rack, such that no separate controllers have to beprovided in the data center system, thus reducing implementationcomplexity.

The data center system may comprise a plurality of server racksseparating the hot aisle from the cold aisle. The data center system mayfurther comprise a plurality of said controllers, wherein each of saidserver racks comprises one of said controllers, and wherein the ceilingelement and the second perforated floor element are responsive to eachindividual controller. This has the advantage each server rack cantrigger the activation of the AC unit, for instance when it is detectedthat the servers in a particular rack switch to a mode of high activity,thereby providing a more fine-grained level of control over thetemperature in the data center.

According to another aspect of the present invention, there is provideda server rack for use in the data center system according to anembodiment of the present invention, the server rack comprising acontroller arranged to toggle the data center system between a firstconfiguration in which the ceiling element is opened and the secondperforated floor element is closed; and a second configuration in whichthe ceiling element is closed and the second perforated floor element isopened.

The server rack may further comprise at least one temperature sensorcoupled to the controller for sensing the temperature in the hot aisleor cold aisle, as previously explained.

In accordance with yet another aspect of the present invention, there isprovided a method of controlling the temperature in a data center systemcomprising a cold aisle comprising a first perforated floor element; ahot aisle comprising a second perforated floor element and a ceilingelement that each can be opened and closed; a server rack comprising aplurality of servers separating the cold aisle from the hot aisle; andan air conditioning unit having an input coupled to the hot aisle viathe ceiling element and an output coupled to the cold aisle via thefirst perforated floor element; the method comprising recirculating airfrom the hot aisle to the cold aisle via the second perforated tile andthe first perforated tile whilst keeping the ceiling element closed; andupon a parameter indicative of server activity reaching a definedthreshold, recirculating air from the hot aisle to the cold aisle viathe ceiling element and the air conditioning unit whilst keeping secondperforated tile closed. This ensures an energy efficient operation ofsuch a data center, as explained in more detail above.

In an embodiment, the method further comprises generating a controlsignal at the server rack to control said recirculation.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the following drawings, in which:

FIG. 1 schematically depicts a data center system according to anembodiment of the present invention in a first configuration;

FIG. 2 schematically depicts a data center system according to anembodiment of the present invention in a second configuration;

FIG. 3 schematically depicts an aspect of a data center system accordingto an embodiment of the present invention; and

FIG. 4 shows a flow chart of a method of controlling the temperature ina data center according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and arenot drawn to scale. It should also be understood that the same referencenumerals are used throughout the Figures to indicate the same or similarparts.

In the context of the present application, where embodiments of thepresent invention constitute a method, it should be understood that sucha method is a process for execution by a computer, i.e. is acomputer-implementable method. The various steps of the method thereforereflect various parts of a computer program, e.g. various parts of oneor more algorithms.

FIG. 1 schematically depicts a data center system according to anembodiment of the present invention in a first configuration. The datacenter system is placed in a computer room 10 having a raised floor 20and a ceiling 30 such as a suspended ceiling. The clearance 26 betweenthe raised floor 20 and the floor of the computer room 10 defines afirst conduit and the clearance 36 between the ceiling 30 and theceiling of the computer room 20 defines a second conduit. The firstconduit is used to transport cool air produced by the air conditioningunit 70 to the cold aisles 40 through first perforated floor elements 22in the raised floor 20. The second conduit is used to transport hot airfrom the hot aisles 50 through ceiling elements 32 to the airconditioning unit 70 for cooling. The air circulated in this manner isindicated by the solid arrows in FIG. 1. It is noted for the avoidanceof doubt that the air conditioning unit 70 may be placed in any suitablelocation, e.g. inside or outside the computer room 10.

As is well-known per se, rows of server racks 60 separate the coldaisles 40 from the hot aisles 50. The server racks typically compriseone or more servers 62 having a front panel facing a cold aisle 40 and aback panel facing a hot aisle 50. Each server 62 typically comprises atleast one fan that forces air from the cold aisle 40 through the server62 towards the hot aisle 50 in order to cool the one or more processingelements and other elements generating heat in the server 62. This isindicated by the solid arrows through the server racks 60.

In accordance with an embodiment of the present invention, the hotaisles further comprise a configurable second perforated floor element24 that forms part of the raised floor 20. The configurable secondperforated floor element 24 can be switched between a firstconfiguration in which the perforations are closed, as shown in FIG. 1,and a second configuration in which the perforations are opened, as willbe explained in more detail later. Similarly, the ceiling elements 32are configurable ceiling elements that can be switched between a firstconfiguration in which the ceiling element is opened, as shown in FIG.1, and a second configuration in which the ceiling element is closed, aswill be explained in more detail later.

The specific implementation of the first and second perforated floorelements 22 and 24 and the ceiling element 32 is not particularlylimited. Any suitable implementation may be chosen. For instance, thefirst and second perforated floor elements 22 and 24 may be perforatedfloor tiles, grates and so on. Similarly, the ceiling element 32 may bea perforated ceiling tile, a grate forming part of the ceiling 30, ormay be a valve-like element mounted on a single server rack 60 or onopposing server racks 60, with the valve being arranged to expose anopening in the ceiling 30 in the first configuration and cover thisopening in the second configuration of the ceiling element 30. It shouldbe understood that any element that can be used to configurably coverand uncover an opening in the raised floor 20 and the ceiling 30 may beused for such a purpose.

The data center system further comprises a plurality of temperaturesensors for monitoring the temperature in at least one of the coldaisles 40 and the hot aisles 50. For instance, in an embodiment, thedata center system may comprise a temperature sensor 66 in each of thecold aisles 40. In an alternative embodiment, the data center system maycomprise a temperature sensor 68 in each of the hot aisles 50. In yetanother embodiment, which is shown in FIG. 1, the data center system maycomprise a temperature sensor 66 in each of the cold aisles 40 as wellas a temperature sensor 68 in each of the hot aisles 50. The temperaturesensors are coupled to a controller 64 arranged to toggle the secondperforated floor element 24 and the ceiling element 32 between the firstconfiguration in which the ceiling element 32 is opened and the secondperforated floor element 24 is closed and the second configuration inwhich the ceiling element 32 is closed and the second perforated floorelement 24 is opened. The first configuration is shown in FIG. 1 and thesecond configuration is shown in FIG. 2.

In the second configuration shown in FIG. 2, the air conditioning unit70 is bypassed and the air forced through the server racks 60 by thefans in the servers 62 from a cold aisle 40 to a hot aisle 50 isrecirculated through a configurable second perforated floor element 24into the first conduit 26 and into a cold aisle 40 through a firstperforated floor element 22.

The switching between the first configuration in FIG. 1 and the secondconfiguration in FIG. 2 is performed to avoid the air conditioning unit70 operating at reduced efficiencies, which for instance occurs if theair temperature of the air transported from the hot aisles 50 to the airconditioning unit 70 is below a certain temperature, e.g. 45° C. Thismay indicate that the servers 62 operate below full capacity, such thatthe cooling of the servers 62 in the server racks 60 is less critical,such that less cold air, i.e. the air from the hot aisles 50 may be usedto cool the servers 62 without having to cool this air using the airconditioning unit 70. If this is the case, the data center system isconfigured as shown in FIG. 2, in which the air conditioning unit 70 isbypassed and the air from the hot aisles 50 is fed back to the coldaisles 40 through the opened second perforated floor element 24 and thefirst perforated floor element 22.

To this end, the controllers 64 may directly or indirectly control thesecond perforated floor element 24 and the ceiling element 32 inresponse to one or more temperature sensors 68 in the hot aisles 50, asindicated by the dashed arrows in FIG. 1 and FIG. 2. In an embodiment,the controllers 64 of the server racks 60 backing the hot aisle 50 arein direct control of the second perforated floor elements 24 and theceiling elements 32 of the hot aisles 50.

The controllers 64 may be interconnected by a network (not shown for thesake of clarity) that is further conductively coupled to the secondperforated floor elements 24 and the ceiling elements 32. Upon one ofthe controllers 64 signaling that the temperature in a hot aisle 50 hasreached a critical value, i.e. a defined threshold, the ceiling elements32 will be switched to an opened state and the second perforated floorelements 24 will be switched to a closed state as shown in FIG. 1 suchthat the air from the hot aisles 50 is fed to the air conditioning unit70.

In the absence of such a signal on the network, the ceiling elements 32will be switched to a closed state and the second perforated floorelements 24 will be switched to an opened state as shown in FIG. 2 suchthat the air from the hot aisles 50 is fed back into the cold aisles 40,bypassing the air conditioning unit 70. In an embodiment, the airconditioning unit 70 is also responsive to the control signal generatedby the controllers 64 such that upon detection of the control signal theair conditioning unit 70 may switch from a standby to an active mode.

In another embodiment, which is particularly preferable, each of thecontrollers 64 is responsive to a temperature sensor 66 in one of thecold aisles 40, such that the controllers 64 can switch from the secondconfiguration in FIG. 2 in which the air conditioning unit 70 isbypassed to the first configuration in FIG. 1 in which the airconditioning unit 70 is included in the air flow from the hot aisles 50to the cold aisles 40. This is for instance advantageous if the servers62 must be cooled by air not exceeding a defined further temperature inorder to guarantee that the servers 62 will not overheat. In anembodiment, this defined further temperature threshold may be 28° C.

In yet another embodiment, the each of controllers 64 may be responsiveto a temperature sensor 66 in one of the cold aisles 40 as well as to atemperature sensor 68 in one of the hot aisles 50 such that theconfiguration of the data center system may be switched between thefirst configuration of FIG. 1 and the second configuration of FIG. 2when at least one of the temperature sensors 66 and 68 indicates thatthe aforementioned defined temperature threshold has been reached, suchthat the air conditioning unit 70 can be activated and included in theair recirculation path from the hot aisles 50 and the cold aisles 40 asexplained in more detail above.

At this point, it is noted that in the above detailed description, thecontrollers 64 and temperature sensor 66 and/or temperature sensor 68are shown as forming part of a server rack 60. It should however beunderstood that this is by way of non-limiting example only. It isequally feasible to provide at least one of the controller 64, thetemperature sensor 66 and/or temperature sensor 68 as separatecomponents of the data center system without departing from theteachings of the present invention.

FIG. 3 schematically depicts an aspect of the data center systemaccording to an embodiment of the present invention. The data centersystem may comprise a network 80 connecting the various controllers 64of the server racks 60 to a master controller 90 that controls the heceiling elements 32 and the second perforated floor elements 24 in thehot aisles 50, and may additionally control the air conditioning unit 70in another embodiment. The master controller may toggle the ceilingelements 32 and the second perforated floor elements 24 in the hotaisles 50 between the first configuration and the second configurationin the presence or absence of a control signal from the controllers 64signaling that the temperature in one of the hot aisles 50 and/or one ofthe cold aisles 40 has reached a defined threshold, as previouslyexplained.

FIG. 4 schematically depicts an embodiment of the method of the presentinvention. Upon initialization of the data center system in step 110, itis checked in step 120 if the temperature in the cold aisles 40 and/orthe hot aisles 50 is below a defined threshold. Preferably, it ischecked if the temperature in the cold aisles 40 is below a definedthreshold, e.g. 28° C. If this is the case, the method proceeds to step130 in which the ceiling elements 32 and the second perforated floorelements 24 are switched to the second configuration in which theceiling elements 32 are closed and the second perforated floor elements24 are opened to bypass the air conditioning unit 70. If this is not thecase, the method proceeds to step 140 in which the ceiling elements 32and the second perforated floor elements 24 are switched to the firstconfiguration in which the ceiling elements 32 are opened and the secondperforated floor elements 24 are closed to include the air conditioningunit 70 in the air recirculation from the hot aisles 50 to the coldaisles 40. It is checked in step 150 if operation of the data centersystem is to continue. If so, the method returns to step 110. Otherwise,the method terminates in step 160. In an embodiment, the decision makingprocess in step 120 is triggered by a control signal generated at theserver rack 60 to control said recirculation, as has been explained inmore detail above.

In the above detailed description, the controllers 64 are responsive toone of more temperature sensors 66 and/or 68. However, it should beunderstood that alternative embodiments in which the controllers 64 areresponsive to another parameter indicative of server activity areequally feasible. For instance, the controllers 64 may monitor theactivity or power consumption of the servers 62 in an associated serverrack 60, and generate the control signal to toggle the configuration ofthe data center system between the first configuration and the secondconfiguration upon the activity or power consumption of the servers 62reaching a defined threshold instead. Other suitable parameters will beapparent to the skilled person.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. Accordingly,the appended claims are intended to encompass all such modifications andchanges as fall within the true spirit and scope of this invention.

1. A data center system comprising: a cold aisle comprising a firstperforated floor element; a hot aisle comprising a second perforatedfloor element that can be opened and closed and a ceiling element thatcan be opened and closed; a server rack comprising a controller and aplurality of servers separating the cold aisle from the hot aisle; andan air conditioning unit having an input coupled to the hot aisle viathe ceiling element and an output coupled to the cold aisle via thefirst perforated floor element wherein the controller is configured totoggle the hot aisle between: (a) a first configuration in which theceiling element is opened and the second perforated floor element isclosed and (b) a second configuration in which the ceiling element isclosed and the second perforated floor element is opened, wherein thefirst configuration enables circulation of air from the from the hotaisle to the cold aisle via the air conditioning unit, and wherein thesecond configuration enables circulation of air from the from the hotaisle to the cold aisle without the circulated air passing through theair conditioning unit.
 2. The system of claim 1, wherein the controlleris arranged to (i) transition the hot aisle from the first configurationto the second configuration in response to a determination that atemperature in the cold aisle has transitioned from at or above adefined first threshold temperature to below the first thresholdtemperature and (ii) transition the hot aisle from the secondconfiguration to the first configuration in response to a determinationthat the temperature in the cold aisle has transitioned from below thefirst threshold temperature to at or above the first thresholdtemperature.
 3. The system of claim 2, wherein the defined firstthreshold temperature is 28° C.
 4. The system of claim 1, wherein thecontroller is arranged to (i) transition the hot aisle from the firstconfiguration to the second configuration in response to a determinationthat a temperature in the hot aisle has transitioned from at or above adefined second threshold temperature to below the second thresholdtemperature and (ii) transition the hot aisle from the secondconfiguration to the first configuration in response to a determinationthat the temperature in the hot aisle has transitioned from below thesecond threshold temperature to at or above the second thresholdtemperature.
 5. The system of claim 4, wherein the defined secondthreshold temperature is 45° C.
 6. (canceled)
 7. The system of claim 4,wherein the server rack comprises a temperature sensor coupled to thecontroller, wherein the controller is responsive to the temperaturesensor, and wherein the temperature sensor is at a back of the serverrack and is configured to sense the temperature in the hot aisle.
 8. Thesystem of claim 2, wherein the server rack comprises a temperaturesensor coupled to the controller, wherein the controller is responsiveto the temperature sensor, and wherein the temperature sensor is at afront of the server rack and is configured to sense the temperature inthe cold aisle. 9-11. (canceled)
 12. A server rack disposed in a datacenter system, said server rack comprising: a controller; and aplurality of servers, wherein the data center system comprises a coldaisle and a hot aisle, wherein the cold aisle comprises a firstperforated floor element. wherein the hot aisle comprises a secondperforated floor element that can be opened and closed and a ceilingelement that can be opened and closed, wherein the controller separatesthe cold aisle from the hot aisle, wherein the controller is configuredto toggle the hot aisle between: (a) a first configuration in which theceiling element is opened and the second perforated floor element isclosed and (b) a second configuration in which the ceiling element isclosed and the second perforated floor element is opened, wherein thefirst configuration enables circulation of air from the from the hotaisle to the cold aisle via the air conditioning unit, and wherein thesecond configuration enables circulation of air from the from the hotaisle to the cold aisle without the circulated air passing through theair conditioning unit. 13-15. (canceled)
 16. The server rack of claim12, wherein the controller is arranged to (i) transition the hot aislefrom the first configuration to the second configuration in response toa determination that a temperature in the cold aisle has transitionedfrom at or above a defined first threshold temperature to below thefirst threshold temperature and (ii) transition the hot aisle from thesecond configuration to the first configuration in response to adetermination that the temperature in the cold aisle has transitionedfrom below the first threshold temperature to at or above the firstthreshold temperature.
 17. The server rack of claim 16, wherein theserver rack comprises a temperature sensor coupled to the controller,wherein the controller is responsive to the temperature sensor, andwherein the temperature sensor is at a front of the server rack and isconfigured to sense the temperature in the cold aisle.
 18. The serverrack of claim 16, wherein the defined first threshold temperature is 28°C.
 19. The server rack of claim 12, wherein the controller is arrangedto (i) transition the hot aisle from the first configuration to thesecond configuration in response to a determination that a temperaturein the hot aisle has transitioned from at or above a defined secondthreshold temperature to below the second threshold temperature and (ii)transition the hot aisle from the second configuration to the firstconfiguration in response to a determination that the temperature in thehot aisle has transitioned from below the second threshold temperatureto at or above the second threshold temperature.
 20. The server rack ofclaim 19, wherein the server rack comprises a temperature sensor coupledto the controller, wherein the controller is responsive to thetemperature sensor, and wherein the temperature sensor is at a back ofthe server rack and is configured to sense the temperature in the hotaisle.
 21. The server rack of claim 19, wherein the defined secondthreshold temperature is 45° C.
 22. A method of controlling aircirculation in a data center system, said data center system comprising:a cold aisle comprising a first perforated floor element; a hot aislecomprising a second perforated floor element that can be opened andclosed and a ceiling element that can be opened and closed; a serverrack comprising a controller and a plurality of servers separating thecold aisle from the hot aisle; and an air conditioning unit having aninput coupled to the hot aisle via the ceiling element and an outputcoupled to the cold aisle via the first perforated floor element, saidmethod comprising: toggling, by the controller, the hot aisle between:(a) a first configuration in which the ceiling element is opened and thesecond perforated floor element is closed and (b) a second configurationin which the ceiling element is closed and the second perforated floorelement is opened, wherein the first configuration enables circulationof air from the from the hot aisle to the cold aisle via the airconditioning unit, and wherein the second configuration enablescirculation of air from the from the hot aisle to the cold aisle withoutthe circulated air passing through the air conditioning unit.
 23. Themethod of claim 22, wherein said toggling the hot aisle comprises:determining that a temperature in the cold aisle has transitioned fromat or above a defined first threshold temperature to below the firstthreshold temperature and in response, transitioning the hot aisle fromthe first configuration to the second configuration; and determiningthat the temperature in the cold aisle has transitioned from below thefirst threshold temperature to at or above the first thresholdtemperature and in response, transitioning the hot aisle from the secondconfiguration to the first configuration.
 24. The method of claim 23,wherein the server rack comprises a temperature sensor coupled to thecontroller, wherein the controller is responsive to the temperaturesensor, wherein the temperature sensor is at a front of the server rack,and wherein the method comprises: said temperature sensor sensing thetemperature in the cold aisle.
 25. The method of claim 23, wherein thedefined first threshold temperature is 28° C.
 26. The method of claim22, wherein said toggling the hot aisle comprises: determining that atemperature in the hot aisle has transitioned from at or above a definedsecond threshold temperature to below the second threshold temperatureand in response, transitioning the hot aisle from the firstconfiguration to the second configuration; and determining that thetemperature in the hot aisle has transitioned from below the secondthreshold temperature to at or above the second threshold temperatureand in response, transitioning the hot aisle from the secondconfiguration to the first configuration.
 27. The method of claim 26,wherein the server rack comprises a temperature sensor coupled to thecontroller, wherein the controller is responsive to the temperaturesensor, wherein the temperature sensor is at a back of the server rack,and wherein the method comprises: said temperature sensor sensing thetemperature in the hot aisle.